news: add note about optional "Removed features" section

Most releases don't need a "Removed features" section so don't include
it in the template by default, but leave a reminder in case it is
relevant.
Signed-off-by: NDaniel P. Berrangé <berrange@redhat.com>

docs/news.xml

@@ -30,6 +30,12 @@
     </section>
   </release>
 
+    If relevant for a given release, add a section for removed
+    features too
+
+    <section title="Removed features">
+    </section>
+
      -->
 
 <libvirt>